Metallic bondcoat or alloy with a high gamma/gamma&#39; transition temperature and a component

ABSTRACT

A metallic bondcoat with phases of γ and γ′ is provided. The metallic coating or alloy is nickel based. The metallic coating or alloy has γ and γ′ phases and optionally has β-phase. The new addition in nickel based coating stabilizes the phases γ and γ′ at high temperatures leading to a reduction of local stresses.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International ApplicationNo. PCT/EP2011/069515 filed Nov. 7, 2011 and claims benefit thereof, theentire content of which is hereby incorporated herein by reference. TheInternational Application claims priority to the U.S. application Ser.No. 12/953,531 filed Nov. 24, 2010, the entire contents of which ishereby incorporated herein by reference.

FIELD OF INVENTION

The invention relates to a metallic bondcoat with phases of γ and γ′ acomponent.

SUMMARY OF INVENTION

Components for the hot gas path in gas turbines are made from Ni- or Cobased materials. These materials are optimized for strength and are notable to withstand oxidation and/or corrosion attack at highertemperatures. Therefore, these kinds of materials must be protectedagainst oxidation by MCrAlY-coatings which can be used as bondcoats forthermal barrier coating (TBC) systems as well. In TBS systems, theMCrAlY coating is needed against hot gas attack on one side and on theother side this coating is needed to adhere the TBC to the substrate.Improving such systems against oxidation will lead to increasedbondcoats service temperatures with increased life properties.

To protect the materials against hot corrosion/oxidation, MCrAlY overlaycoatings are coated mainly by low pressure plasma spraying (LPPS), airplasma spraying (APS), electron beam physical vapor deposition (EBPVD),cold spray (CS) or high velocity oxy-fuel (HVOF) process. The MCrAlYcoating is based on nickel and/or cobalt, chromium, aluminum, silicon,rhenium and rare earth elements like yttrium. With increasing bondcoattemperatures, these coatings can fail which can lead to spallation ofthe thermal barrier coating. Therefore, with increasing servicetemperatures, improved coatings are needed to withstand the oxidationattack. Additionally this kind of coatings should have acceptablethermo-mechanical properties. These requests can only be achieved by anoptimized composition of the bond coat.

It is therefore the aim of the invention to solve the above mentionedproblem.

The problem is solved by a metallic coating or an alloy and a componentaccording to the independent claims.

In the dependent claims further amendments are disclosed which can bearbitrarily combined with each other to yield further advantages.

BRIEF DESCRIPTION OF DRAWINGS

It shows

FIG. 1 a turbine blade,

FIG. 2 a gas turbine and

FIG. 3 a list of superalloys.

DETAILED DESCRIPTION OF INVENTION

The figures and the description are only embodiments of the invention.

FIG. 1 shows a perspective view of a rotor blade 120 or guide vane 130of a turbomachine, which extends along a longitudinal axis 121.

The turbomachine may be a gas turbine of an aircraft or of a power plantfor generating electricity, a steam turbine or a compressor.

The blade or vane 120, 130 has, in succession along the longitudinalaxis 121, a securing region 400, an adjoining blade or vane platform 403and a main blade or vane part 406 as well as a blade or vane tip 415.

As a guide vane 130, the vane 130 may have a further platform (notshown) at its vane tip 415.

A blade or vane root 183, which is used to secure the rotor blades 120,130 to a shaft or disk (not shown), is formed in the securing region400.

The blade or vane root 183 is designed, for example, in hammerhead form.Other configurations, such as a fir-tree or dovetail root, are possible.

The blade or vane 120, 130 has a leading edge 409 and a trailing edge412 for a medium which flows past the main blade or vane part 406.

In the case of conventional blades or vanes 120, 130, by way of examplesolid metallic materials, in particular superalloys, are used in allregions 400, 403, 406 of the blade or vane 120, 130.

Superalloys of this type are known, for example, from EP 1 204 776 B1,EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949.

The blade or vane 120, 130 may in this case be produced by a castingprocess, also by means of directional solidification, by a forgingprocess, by a milling process or combinations thereof.

Workpieces with a single-crystal structure or structures are used ascomponents for machines which, in operation, are exposed to highmechanical, thermal and/or chemical stresses.

Single-crystal workpieces of this type are produced, for example, bydirectional solidification from the melt. This involves castingprocesses in which the liquid metallic alloy solidifies to form thesingle-crystal structure, i.e. the single-crystal workpiece, orsolidifies directionally.

In this case, dendritic crystals are oriented along the direction ofheat flow and form either a columnar crystalline grain structure (i.e.grains which run over the entire length of the workpiece and arereferred to here, in accordance with the language customarily used, asdirectionally solidified) or a single-crystal structure, i.e. the entireworkpiece consists of one single crystal. In these processes, atransition to globular (polycrystalline) solidification needs to beavoided, since non-directional growth inevitably forms transverse andlongitudinal grain boundaries, which negate the favorable properties ofthe directionally solidified or single-crystal component.

Where the text refers in general terms to directionally solidifiedmicrostructures, this is to be understood as meaning both singlecrystals, which do not have any grain boundaries or at most havesmall-angle grain boundaries, and columnar crystal structures, which dohave grain boundaries running in the longitudinal direction but do nothave any transverse grain boundaries. This second form of crystallinestructures is also described as directionally solidified microstructures(directionally solidified structures).

Processes of this type are known from U.S. Pat. No. 6,024,792 and EP 0892 090 A1.

The blades or vanes 120, 130 may likewise have coatings protectingagainst corrosion or oxidation, e.g. MCrAlX (M is at least one elementselected from the group consisting of iron (Fe), cobalt (Co), nickel(Ni), X is an active element and represents yttrium (Y) and/or siliconand/or at least one rare earth element, or hafnium (Hf)). Alloys of thistype are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 orEP 1 306 454 A1.

The density is preferably 95% of the theoretical density.

A protective aluminum oxide layer (TGO=thermally grown oxide layer)forms on the MCrAlX layer (as an intermediate layer or an outermostlayer).

It is also possible for a thermal barrier coating, consisting forexample of ZrO₂, Y₂O₃—ZrO₂, i.e. unstabilized, partially stabilized orfully stabilized by yttrium oxide and/or calcium oxide and/or magnesiumoxide and/or one or more of rare earth element (lanthanum, gadolinium,yttrium, etc.), which is preferably the outermost layer, to be presenton the MCrAlX.

The thermal barrier coating covers the entire MCrAlX layer. Columnargrains are produced in the thermal barrier coating by means of suitablecoating processes, such as for example electron beam physical vapordeposition (EB-PVD).

Other coating processes are conceivable, for example atmospheric plasmaspraying (APS), LPPS, VPS, solution precursor plasma spray (SPPS) orCVD. The thermal barrier coating may include porous grains which havemicrocracks or macrocracks for improving its resistance to thermalshocks. The thermal barrier coating is therefore preferably more porousthan the MCrAlX layer.

The blade or vane 120, 130 may be hollow or solid in form. If the bladeor vane 120, 130 is to be cooled, it is hollow and may also havefilm-cooling holes 418 (indicated by dashed lines).

FIG. 2 shows, by way of example, a partial longitudinal section througha gas turbine 100.

In the interior, the gas turbine 100 has a rotor 103 which is mountedsuch that it can rotate about an axis of rotation 102, has a shaft 101and is also referred to as the turbine rotor.

An intake housing 104, a compressor 105, a, for example, toroidalcombustion chamber 110, in particular an annular combustion chamber,with a plurality of coaxially arranged burners 107, a turbine 108 andthe exhaust-gas housing 109 follow one another along the rotor 103.

The annular combustion chamber 110 is in communication with a, forexample, annular hot-gas passage 111, where, by way of example, foursuccessive turbine stages 112 form the turbine 108.

Each turbine stage 112 is formed, for example, from two blade or vanerings. As seen in the direction of flow of a working medium 113, in thehot-gas passage 111 a row of guide vanes 115 is followed by a row 125formed from rotor blades 120.

The guide vanes 130 are secured to an inner housing 138 of a stator 143,whereas the rotor blades 120 of a row 125 are fitted to the rotor 103for example by means of a turbine disk 133.

A generator (not shown) is coupled to the rotor 103.

While the gas turbine 100 is operating, the compressor 105 sucks in air135 through the intake housing 104 and compresses it. The compressed airprovided at the turbine-side end of the compressor 105 is passed to theburners 107, where it is mixed with a fuel. The mix is then burnt in thecombustion chamber 110, forming the working medium 113. From there, theworking medium 113 flows along the hot-gas passage 111 past the guidevanes 130 and the rotor blades 120. The working medium 113 is expandedat the rotor blades 120, transferring its momentum, so that the rotorblades 120 drive the rotor 103 and the latter in turn drives thegenerator coupled to it.

While the gas turbine 100 is operating, the components which are exposedto the hot working medium 113 are subject to thermal stresses. The guidevanes 130 and rotor blades 120 of the first turbine stage 112, as seenin the direction of flow of the working medium 113, together with theheat shield bricks which line the annular combustion chamber 110, aresubject to the highest thermal stresses.

To be able to withstand the temperatures which prevail there, they canbe cooled by means of a coolant.

Substrates of the components may likewise have a directional structure,i.e. they are in single-crystal form (SX structure) or have onlylongitudinally oriented grains (DS structure).

By way of example, iron-based, nickel-based or cobalt-based superalloysare used as material for the components, in particular for the turbineblade or vane 120, 130 and components of the combustion chamber 110.

Superalloys of this type are known, for example, from EP 1 204 776 B1,EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949.

The guide vane 130 has a guide vane root (not shown here) facing theinner housing 138 of the turbine 108 and a guide vane head at theopposite end from the guide vane root. The guide vane head faces therotor 103 and is fixed to a securing ring 140 of the stator 143.

A new modified coating was developed which fulfils the requirementsdescribed above. This coating has a good long term life, acceptablemechanical properties and improved oxidation resistance. This is basedon the presence of tantalum (Ta) in a nickel based alloy but preferablywithout rhenium (Re). Tantalum (Ta) stabilizes the formation of a threephase system (γ′/γ/β) with a high γ′/γ transition temperature. This willreduce the local stresses as well because tantalum (Ta) will stabilizethe high transition temperatures of γ′ which is higher than the bondcoatservice temperature.

Therefore there is preferably no need for hafnium (Hf), silicon (Si) orzirconium (Zr) or any melting depressant (B) in the coating.

Very good results show the following elemental composition for gettingthe proposed 3-phase-system with increased γ′ transition temperatures:Ni-23Co-20Cr-10Al-4.5Ta.

A composition (Ni-25Co-17Cr-10Al-1.5Re-Y) which contains rhenium (Re)instead of tantalum (Ta) has a lower γ′/γ transition temperature becauseno tantalum (Ta) is added.

The bondcoat is preferably a nickel (Ni) based super alloy with additionof cobalt (Co), chromium (Cr), aluminum (Al) and optionally yttrium (Y)which is preferably consisting of these elements.

Very preferably it is a MCrAl(X) alloy, with M=Ni, Co.

Preferably the alloy contains no molybdenum (Mo), and/or no tungsten (W)and/or no columbium (Nb) and/or no platinum (Pt).

1.-15. (canceled)
 16. A metallic coating or alloy that is nickel basedcomprising γ and γ′ phases and/or β-phase, comprising: a tantalum thatis in a range between 3.0 wt % to 6.0 wt %, or is in a range between 3.5wt % to 5.5 wt %, or is 4.5 wt %.
 17. The metallic coating or alloyaccording to claim 16, wherein amount of cobalt is in a range between 21wt % to 25 wt %, or is in a range between 22 wt % to 23.5 wt %, or is 23wt %.
 18. The metallic coating or alloy according to claim 16, whereinamount of chromium is in a range between 18 wt % to 22 wt %, or is in arange between 19 wt % to 21 wt %, or is 20 wt %.
 19. The metalliccoating or alloy according to claim 16, wherein the metallic coating oralloy comprises no Yttrium.
 20. The metallic coating or alloy accordingto claim 16, wherein amount of aluminum is in a range between 8 wt % to12 wt %, or is in a range between 9 wt % to 11 wt %, or is 10 wt %. 21.The metallic coating or alloy according to claim 16, wherein amount ofyttrium is in a range between 0.1 wt % to 0.7 wt %.
 22. The metalliccoating or alloy according to claim 16, wherein the metallic coating oralloy comprises no rhenium.
 23. The metallic coating or alloy accordingto claim 16, wherein the metallic coating or alloy comprises 0.1 wt % to2 wt % rhenium.
 24. The metallic coating or alloy according to claim 16,wherein the metallic coating or alloy is a MCrAl alloy with M=nickeland/or cobalt.
 25. The metallic coating or alloy according to claim 16,wherein the metallic coating or alloy has a higher γ′/γ transitioncompared to a NiCoCrAl alloy or coating with rhenium and withouttantalum.
 26. A coating or alloy, comprising: a tantalum according toclaim 16; no silicon; no hafnium; no zirconium; no tungsten; noplatinum; and/or no melting depressant.
 27. The coating or alloyaccording to claim 26, wherein the coating or alloy comprises β-phasethat is at least 5 vol %.
 28. The coating or alloy according to claim26, wherein the coating or alloy comprises nickel, cobalt, aluminum,chromium, tantalum and optionally Yttrium.
 29. The coating or alloyaccording to claim 26, wherein the coating or alloy comprises no iron.30. A component, comprising: a metallic coating according to claim 16.